1. Field of the Invention
The present invention relates to a method of preventing corrosion degradation using Ni or Ni-alloy plating.
2. Description of the Related Art
In recent years, a commercial nuclear reactor that has been operated all over the world includes a pressurized water reactor and a boiling water reactor developed in the U.S.A, a high-temperature gas cooling reactor developed in the U.K. and a pressurized heavy water reactor developed in Canada. All nuclear power plants in Korea, except Wolseong nuclear power plant, are provided with pressurized water reactors. The pressurized water reactor (PWR) uses lowly-concentrated uranium containing approximately 2˜5% of uranium 235 as fuel and uses water (light water) as a coolant or moderator. The water is made not to boil inside a nuclear reactor by pressurizing a primary cooling system at a pressure of approximately 150 atms. Water heated to high temperature is sent to a steam generator and is changed into steam through heat exchange with secondary side water. The heat-exchanged primary side water is returned to the nuclear reactor and is heated, and then the heated water is sent to the steam generator again. This process is repeatedly performed.
One of the accidents often occurring in the pressurized water nuclear power plant is leakage in a heat transfer tube of a steam generator. It is determined that the leakage in the heat transfer tube of the steam generator occurs due to two or more causes. One of the causes of the leakage is that the thickness of the heat transfer tube is decreased. As the result of examining eddy currents of the heat transfer tube, it was found that the heat transfer tube becomes thinner due to friction between the heat transfer tube and a tube support plate or an AVB (anti-vibration bar), wherein the friction results from the vibration of the heat transfer tube due to the flow of a fluid. Currently, since this tube wear phenomenon occurring due to the vibration of the tube caused by the flow of fluid, unlike the degradation of the tube due to corrosion, can be improved by upgrading the design of the steam generator. Due to the upgrade of the design of the steam generator, the vibration of the tube caused by the flow of fluid is remarkably reduced, but is still one main cause for the degradation to the heat transfer tube.
As another cause for the leakage in the heat transfer tube, various types of corrosion such as stress corrosion cracking, pitting corrosion, etc. occur in a top of a tubesheet, that is, including an expansion transition region of the heat transfer tube (the top of the tubesheet and the expansion transition region are substantially consistent with each other) due to the sludge piled up on the top of the tubesheet and high residual stress around the expansion transition region of the tube. The sludge made of various metal oxides including iron oxides, etc. and metals including copper, etc piled up on the top of the tubesheet changes chemical and thermal environments between sludge and the heat transfer tube into an environment aggravating the corrosion and results in generating tensile stress which may cause the stress corrosion cracking in the heat transfer tube by partially transforming the heat transfer tube due to tenting occurring due to corrosion oxidation of the tubesheet between the heat transfer tube and the tubesheet made of carbon steel. Therefore, in order to alleviate the corrosion degradation in the expansion transition region of the heat transfer tube (the top of the tubesheet), it is very important to remove the sludge during the operation of a nuclear power plant. The method of removing the sludge is called a sludge lance absorption method (Korean Unexamined Patent Publication No. 1981-0000034). However, even in the case where the sludge is not almost accumulated in the top of the tubesheet in the nuclear power plant in operation, the stress corrosion cracking occurs. The reason for this is that stress applied to the expansion transition region of the heat transfer tube during the operation of the nuclear power plant, or the corrosive environment of primary cooling water and secondary cooling water, and metallurgical stress corrosion cracking sensitivity of the heat transfer tube are compositely acted.
Therefore, a material of the heat transfer tube is a major factor in the cracking of the heat transfer tube. Currently, Inconel 600 alloy mainly containing nickel is being used as the material of the heat transfer tube of a steam generator in the nuclear power plant. The Inconel 600 alloy is excellent in mechanical properties and corrosion resistance, and thus is used as the material of the heat transfer tube of the steam generator in the pressurized water nuclear power plant, but the heat transfer tube is vulnerable to the stress corrosion cracking under operating conditions at primary and secondary sides of the steam generator, and thus intergranular corrosion and the stress corrosion cracking frequently occur under the operating condition. In particular, the intergranula corrosion and the stress corrosion cracking more frequently occur in the material of the heat transfer tube at the secondary side.
The intergranular corrosion is described as follows. When austenitic Ni-base alloys are heated to 500˜800° C., carbides (Cr23C6) are formed on the grain boundary thereof, and the amount of chromium (Cr) existing at the portion adjacent to the grain boundary is reduced, thereby forming a Cr depletion region. A process of making this state is referred to as sensitization treatment. The sensitization-treated alloys are immersed into a corrosive solution, the Cr depletion region is remarkably corroded, resulting in disintegration of grains. This phenomenon is referred to as intergranular corrosion.
The stress corrosion cracking is a phenomenon that a metallic material under the tensile stress becomes brittle and easily broken under a specific combination of the material and the corrosion environment. The stress corrosion cracking occurs only when three conditions such as the material, the environment, and the stress satisfy the specific condition. In general, a material having excellent corrosion resistance has a passivation layer formed on the surface thereof. However, the passivation layer is partially broken due to external causes, and thus becomes a starting point for the pitting or the stress corrosion cracking. Stress concentration is partially increased, and the corrosive solution contributes to the propagation of the stress corrosion cracking, thereby accelerating the cracking.
The intergranular corrosion or the stress corrosion cracking of the heat transfer tube of the steam generator causes a leakage accident of the primary cooling water and unscheduled trip of the plant, and becomes a direct cause for repair of the broken heat transfer tube and finally the replacement of the steam generator itself, thereby incurring an enormous economic loss.
Accordingly, in order to prevent an accident and a loss due to the corrosion and the stress corrosion cracking of the heat transfer tube of the steam generator in the nuclear power plant, or in order to prevent deterioration occurring in various materials of parts through which cooling water passes so as to reduce the amount of the sludge primarily causing the corrosion degradation of the heat transfer tube of the steam generator, researches on the development of an alloy capable of substitution, proper chemical water treatment (secondary water treatment) and the improvement of a processing operation of the steam generator are required.
Conventionally, Korean Patent Registration No. 415265 discloses a method of suppressing the stress corrosion cracking at the secondary side of the heat transfer tube of the steam generator in the nuclear power plant, in which a compound selected from the group consisting of cerium boride, lanthanum boride, and a mixture thereof is supplied to secondary cooling water, and by which the resistance to the stress corrosion cracking of the heat transfer tube can be improved by three times or more, and by two times or more compared to a conventional corrosion inhibitor.
Further, Korean Patent Registration No. 609590 discloses an inhibitor containing nickel boride and a method of suppressing the corrosion and the stress corrosion cracking at a secondary side of the heat transfer tube of the steam generator in the nuclear power plant, using the inhibitor. In this method, nickel boride suppresses the stress corrosion cracking of a specimen simulating the heat transfer tube of the steam generator in the nuclear power plant and increases the corrosion resistance by reducing corrosion current density and the thickness of an oxide film. However, the expansion transition region in the top of the tubesheet of the heat transfer tube is still degraded and an effort to prevent the corrosion degradation is continuously exerted.
Meanwhile, the degraded heat transfer tube is discarded by plugging the tube or is reused by sleeving the tube. As a repairing technology for sleeving the tube, a technology of plating the inner portion of the tube, adjacent to a degraded portion, with Ni or Ni-alloy has been developed ((a) Larue, F., “Nickel plating S.G. tubing repair”, Proc. of the 1991 JAIF international conference on water chemistry in nuclear power plants, 1989 pp. 163-167; (b) Michaut, B., “Nickel electroplating as a remedy to steam generator tubing PWSCC”, Proc. of the 6th international symposium on environmental degradation of materials in nuclear power systems-water reactors, 1993 pp. 713-719; (c) Stubbe, J. et al., “Repairing cracked tubes with Nickel plating”, Nuclear Engineering International, Vol. 34 (1989) pp. 31-33; (d) Gonzalez, F., Brennenstuhl, A. M., Palumbo, G., Erb, U., and Lichtenberger, P. C., “Electrodeposited Nanostructured Nickel for In-Situ Nuclear Steam Generator Repair”, Materials Science Forum, Vol. 225-227 (1996) pp. 831-836). Therefore, it has been found that the Ni plated tube or Ni-alloy plated tube has excellent corrosion resistance to the corrosion degradation and to the pitting, the stress corrosion cracking (SCC), etc.
More specifically, in the Ni plating, structural integrity cannot be granted to the degraded heat transfer tube in mechanical properties, but it is possible to prevent leakage or the crack from growing by plating an adjacent part including a defective portion in the Ni-alloy plating, since excellent mechanical properties can be bestowed to the degraded heat transfer tube, it is possible to prevent leakage in the degraded heat transfer tube and acquire the structural integrity, thereby performing the sleeved tube having a circumferential cracking defect as a heat transfer tube.